Brazil’s dead monkeys are connected to a human disease outbreak

While most of the world is focusing on Zika, Brazil is currently dealing with another serious disease outbreak. People first noticed signs of trouble when monkeys started dropping dead in the forest. Soon people started getting sick and dying as well.

The cause of over 600 monkey deaths and 173 suspicious human deaths is yellow fever. Yellow fever has not been a major issue in Brazil since the 1940s, when mass vaccination and mosquito control campaigns nearly eradicated the disease. Since then, only a handful of human cases happen each year. These cases are sylvatic or “jungle” cases as they occur when people are bitten by mosquitoes in areas where mosquitoes are known to be able to transmit yellow fever. Outbreaks normally die out on their own due to spontaneous reasons or as people in affected areas get vaccinated and the mosquitoes run out of susceptible humans. This current outbreak has spread far beyond the “usual” size.

A graphic demonstrating how humans can spread yellow fever in jungle areas, suburban areas and urban areas. Courtesy WHO.

The spread of yellow fever. Courtesy WHO.

This outbreak was first noticed in humans in December in people living in rural areas of the Brazilian state of Minas Gerais, a mainly agricultural area north of Sao Paulo and Rio de Janiero. Yellow fever causes fever, headache, jaundice, muscle pain, nausea, vomiting, and fatigue. Most people with yellow fever do not experience symptoms and do not know they are infected. However, some patients progress to a serious second phase of infection where they develop life-threatening symptoms. Half of the patients who go on to this second phase die from their infection.

Map of Brazil showing large areas across the country that were at risk for yellow fever in 2013 compared to a very small area on the west coast which is at risk for 2017. Courtesy WHO.

Map of areas at risk for yellow fever transmission in 2013 and 2017 in Brazil. Courtesy WHO.

According to Brazilian officials, there have been over 1000 reported cases to date, compared to the handful of cases that normally occur each year. The concern is that this outbreak will spread to urban areas where Aedes aegypti mosquitoes live. Those are the mosquitoes that can also transmit Zika and dengue and they thrive in urban settings, where they can survive in the smallest containers of standing water. Because many Latin American countries haven’t had large numbers of yellow fever cases in decades, many people have never been vaccinated. All of this could be a recipe for disaster.

Worldwide yellow fever outbreak control efforts are challenged by an ongoing yellow fever vaccine shortage, as outbreaks have recently occurred (and some are still occurring) in Angola, Democratic Republic of the Congo, and Uganda, in addition to Brazil. Officials in Brazil have declared a public health emergency and have been vaccinating millions of people to try to stop the spread.

Falling vaccine rates coupled with more frequent interactions between people and disease vectors means the world may see increasing numbers of outbreaks.

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